Energy and Power in Electric Circuits – Learn
Potential difference is defined as the work done to move a charge against an electric field between two points. The equation relating potential difference and work is:
V=\cfrac { W }{ q } or W=Vq
Where:
V = voltage (in V)
W = work (in J)
q = charge (in C)
The rate at which energy is used or converted into other forms of energy over a period of time is known as its electrical power. Power (P) is the rate at which energy is transformed in a circuit component, and is measured in watts (W). Power is calculated using:
P=IV=\cfrac { E }{ t }
Where:
P = power (in W)
These equations are often rearranged to give the relationship:
W=E=VIt
Example 1:
Calculate the work carried by 4C of charge with a potential difference of 12V:
Answer:
Using: W=Vq
W=12\times 4
W= 48J
Example 2:
A speaker operating on a voltage of 18V draws a current of 3A. Calculate the power used by this speaker:
Answer:
Using P=IV
P=18\times 3
P=54W
Example 3:
A potential difference of 6V is used to produce a current of 150mA in a set of headphones for 10 minutes. How much energy is used in this time?
Answer:
Using: E=VIt
E=6\times 0.15\times 600
E=540J